The present invention relates generally to the field of bus systems and more particularly to an arrangement for coupling bus subscribers and bus masters in a master-slave bus system which has a ring topology and in which the bus subscribers, or slave subscribers, are coupled actively without subscriber addresses.
Bus systems having a ring topology and active subscriber coupling, as shown in German Patent DIN E 19258, generally do not have the facility for removing individual bus subscribers from the overall system or for adding them to it. Up to now, such a capability has been offered only by bus systems having a linear topology and passive subscriber coupling, in accordance with German Patent DIN 19245. The physical characteristics of the latter bus systems allow the connection and disconnection of a subscriber without the whole system being brought to a standstill.
In other types of ring systems, namely computer networks, so-called bypass elements are familiar tools for producing a bridge when a subscriber is removed from the ring. However, these devices are used exclusively for increasing the availability of the ring system, the recovery time of the system being of secondary importance in this case. The situation is different for bus systems because the concern here is not to interrupt data transmission between the interconnected computer or control systems with a wide range of configurations, otherwise it will be necessary to establish not only the data link but also the operation of the control systems all over again.
It is therefore an object of the present invention to provide a coupling arrangement for a master-slave bus system in which, in the event of a disruption, such as in the case of a slave subscriber being coupled, decoupled or experiencing a fault, the rapid and direct driving without subscriber addresses is interrupted only so briefly that the control or computer systems connected via the bus system are not affected.
The foregoing and other objects and advantages of the present invention will appear more clearly hereinafter. In accordance with the present invention there is provided a coupling arrangement for a master-slave bus system in which each slave subscriber in a ring topology is allocated a bypass element. This bypass element is used to make the bus master, when the associated slave subscriber is decoupled and coupled, briefly interrupt direct data interchange with the slave subscribers and perform address interrogation for the purpose of restructuring the protocol execution, to which end the bypass elements have an address store which can be selected only in the event of this address interrogation and is inactive during data interchange.
In the master-slave bus system which is suitable for extremely fast data transmission and in which the bus subscribers are coupled actively without subscriber addresses, the removal, change or failure of one of the slave subscribers causes only a brief restructuring of the protocol execution which, appropriately without feedback, does not affect the connected control or computer systems. After restructuring of the ring system, which is not interrupted physically on account of the bypass elements present according to the invention, data interchange between the bus master and the slave subscribers continues, it being possible for the altered ring topology to be included in the bus master""s protocol execution from the outset after the occurrence of a fault.
The bypass elements can either be arranged physically separate from the respectively associated slave subscriber or they can be integrated in the latter such that it is still possible to exchange the slave subscriber on its own, as is necessary when there is a fault. The crucial factor is that the ring topology of the bus system is retained by virtue of the bypass elements being connected to one another via forward and return data channels.
In order that the slave subscriber can be included in the data interchange of the ring system, it is connected to the allocated bypass element via a reception data line and via a transmission data line. In addition, between each slave subscriber and the associated bypass element, there is a connection via a control line which is used to pass either a decoupling signal or a coupling signal to the bypass element, depending on which state change is currently occurring.
The address stores which can be read by the bus master are each arranged in the bypass elements, but preferably cannot be called up directly from there. Hence, in an expedient refinement of the invention, the address stores of the bypass elements are each connected to the associated slave subscriber by means of an interface so that the address stores can be read only via this slave subscriber when coupled. If the address store cannot be accessed by the bus master because the slave subscriber is not present or has a fault, it is not necessary for the data in the address store of the absent slave subscriber to be additionally evaluated again for protocol execution to continue in the bus master.
Advantageously, the bypasses are constructed using a permanent passive circuit which does not require a specific voltage supply.